The path which an air vehicle must follow during flight, the attitude (the pitch and roll) of the vehicle as well as its speed, are controlled by means of actuators associated to several elements of the vehicle, specifically to its propulsion system (the engines) and to the vehicle control surfaces. In other words, in order for the vehicle to follow a desired path with a desired attitude and speed, it is necessary to generate and send suitable control signals to the actuators, such that the actuation of the propulsion system (including the developed thrust) and the position or orientation of each control surface is suitable so that the vehicle may follow the path as desired. The generation of these signals is usually carried out by an on-board computer system of the vehicle from commands generated by a pilot (which can be on board the vehicle or which the pilot controls remotely from a land control station or a station in another vehicle) by a mission management system or by both.
FIG. 1 shows a flight control system for an air vehicle according to the state of the art. As can be seen, the system includes a mission management module 1 (which can have a stored path, for example in waypoint form, or which can receive flight instructions, for example in waypoint form, preprogrammed maneuvers, or the like, generated by a pilot 1A who may be on board the vehicle or who may transmit instructions to the vehicle from a land control station or a control station in another vehicle).
On the other hand, the flight control system comprises a navigation and guidance subsystem 2 comprising a navigation module 2A—which calculates the ground speed and position of the vehicle with respect to the Earth—and a guidance module 2B, which attempts to correct errors such that the real path and speed adapt to the desired path and speed according to the information supplied to the mission management module. To that end, the guidance module 2B usually comprises control laws, the control variables of which are the errors in the path, for example a PID (proportional, integral and derivative) control system.
The navigation and guidance subsystem receives data regarding the current vehicle conditions from the sensors 3. These sensors may include a GPS system, airspeed detectors, altitude detectors, acceleration and angular velocity detectors (for example, in the form of gyroscopes and accelerometers) as well as magnetometers (which provide a magnetic field measurement and can be used to determine the pitch and roll of the vehicle). The navigation and guidance subsystem usually does not receive the data directly from the sensors 3 but through an estimation module 4 which processes the data from the sensors and provides a series of (supposedly) processed data indicating certain flight conditions, for example the vehicle airspeed conditions, its ground speed, the roll, pitch and position of the vehicle with respect to the Earth, as well as data relating to the state of the actuators 6 controlling the propulsion system (the engines) and the vehicle control surfaces (in the event that the required sensors are arranged).
On the other hand, the flight control system comprises what is usually referred to as a primary control module 5 receiving data (d) with respect to the flight conditions from the estimation module 4, as well as control parameters (p) from the navigation and guidance module 2 (or, in some cases, directly from the mission management module 1). Said control parameters p usually comprise:                a parameter V indicating a desired speed;        a parameter R indicating desired turn characteristics (for example, turning radius, turning speed or angle of roll) (rotation usually relates to the rotation of the projection of the path of the vehicle in a horizontal plane); and        a parameter γ indicating a desired path angle (path angle is usually understood as the angle of the path of the vehicle in the vertical plane with respect to the horizontal plane).        
These three parameters V, R and γ may relate to the Earth or air; the transformation of Earth-related parameters to air-related parameters (or vice versa) can be carried out directly once the speed of the air is known.
Based on these parameters p and data d, the primary control module 5 calculates the signals s for the actuators of the vehicle: these signals determine how the control surfaces will be oriented as well as the power that the vehicle engines will provide. The state of the art includes a large number of systems and algorithms which can be used for generating the control signals (s) from the control parameters (p) and data (d) regarding the state and condition of the vehicle.
The parameters (p) which the primary control module 5 receives may include:
A) In the case of a vehicle which must “automatically” follow a predefined path or route (established, for example, by means of a series of waypoints known by a mission management system, or by means of high-level commands from a pilot):                a desired speed of the vehicle;        a desired path angle (i.e. the angle which the path of the vehicle follows in the vertical plane with respect to the horizontal plane); and        a desired turning speed or turning radius for turns in the horizontal plane.        
(These control parameters are calculated by the navigation and guidance subsystem 2; a large number of systems and algorithms are known in the state of the art for calculating this type of control parameters from the route data which the vehicle has and from data indicating the flight conditions, therefore it is not necessary to herein describe said systems and algorithms in further detail).
B) In the case of a vehicle flying according to basic instructions commanded by a physical pilot:                a desired speed of the vehicle        a desired attitude of the vehicle, i.e. its pitch and angles of roll.        
For at least some of the control parameters (p), or combinations thereof, there are usually limits establishing what is usually referred to as an operational envelope and which are used to prevent the system from accepting values of the control parameters which may represent a danger, for example a reduction of the speed under a minimum speed, a turning radius which may represent excessive stress on parts of the vehicle, etc. These limits may vary over time and be a function of the current flight conditions (represented by the data provided by the sensors 3 and estimation module 4). The limits may further be interrelated, for example the envelope for the turning radius (i.e. the limits between which the turning radius may vary) may depend on the speed, etc.
In unmanned air vehicles, the paths to be followed are many times established beforehand and the mission management module 1 is responsible for generating the control parameters (p); it usually does this through the navigation and guidance subsystem although it is also possible to generate the control parameters without resorting to this subsystem.
In the system shown in FIG. 1, the control parameters can be calculated in the navigation and guidance subsystem 2 taking into consideration the real position of the vehicle with respect to a series of waypoints with the intention that the vehicle follows, in the most reliable manner possible, the originally planned path, which is appropriate for reducing the risk of accidents insofar as it is possible (and furthermore the problem represented by recalculating the routes during the flight). However, unexpected (and often unforeseeable) events often occur, for example changes in the atmospheric conditions, problems in the vehicle equipment, exits from the expected route due to an intervention by the pilot from the land station, etc., causing problems for the vehicle in following the planned path, for example for climbing with the originally planned path angle, for example due to an excess of wind in the flight direction, which would make a greater (aerodynamic) path angle necessary, the necessary power in this case possibly being greater that the available power, or due to the fact that the engines do not allow developing the originally planned power. In these cases, a conventional solution to the problem consists of recalculating the route to be followed, something which however may represent a problem given that it may require a high calculation capacity (which may exceed the capacity of the on-board systems of the vehicle) and certain risks (for example, in the case of an unmanned air vehicle, because the system generating the alternative path may not have relevant data relating to obstacles which may be present in the new path, for example mountains, civil aviation airways, etc.; on the other hand, having such data would entail a great computational cost).
Patent document U.S. Pat. No. 6,493,609 describes an automatic envelope protection system for unmanned air vehicles. Basically, an envelope protection subsystem is intercalated between a navigation system (which may basically produce a series of input control parameters calculated from the data of the expected route, for example waypoints, which the navigation system has) and a control system which must calculate the output control signals for the actuators from said input control parameters. To protect the envelopes, the protection system takes corrective actions to modify the input control parameters before they reach the system calculating the control signals for the actuators; for that purpose it is based, among others, on the data relating to the current state of the vehicle, which allows determining if the vehicle is excessively approaching the limits represented by the envelopes. The attempt is made to maintain the real vehicle conditions within certain limits, not the control parameters which are sent to the control module. The attempt is made to maintain the originally planned path, insofar as it is possible, by applying certain offsets to the actuators for a certain time such that values within the envelope are obtained. The possibility of recalculating the originally planned mission or path so as to offset the alternation of the input control parameters forced by the protection system is provided.
Patent document U.S. Pat. No. 6,711,477 describes a similar system and method for numerically determining the flight envelope. Calculation routines are contemplated which may be complex and require a high calculation capacity.
Patent document US-A-2002/0055809 describes a system in which there is a computer in parallel with the flight control computer which analyzes the risk-entailing conditions using fuzzy logic. The results of the analysis are shown to the pilot in order to take the measurements which he/she considers as appropriate.
Patent document U.S. Pat. No. 6,163,744 describes a system for modifying a flight route as a response to changes in certain parameters. In other words, the flight plan, i.e. the mission, is modified.
It is possible that at least some of the known systems can be used for preventing an air vehicle (manned or unmanned) from exiting its flight envelope. However, this is many times obtained in the known systems by recalculating the route or path to be followed, establishing an alternative route. This may involve several problems: it requires sufficient data and calculation capacity to establish a new “safe” route and/or access to a “preprogrammed” alternative route. It has been considered that it would be desirable to reduce the cases in which it is necessary to recalculate the route or choose another alternative route in order to respect the flight envelope. It has likewise been considered that it would be desirable to obtain that the vehicles substantially adapt to the planned route and that the (possibly temporary) deviations of said route (especially in the horizontal plane) are not greater than which is (strictly) necessary for maintaining the vehicle within its flight envelope. It has further been considered that it would be desirable to achieve these objectives with a system that does not require a high calculation capacity, which is performed in real time and which can be incorporated in the flight control systems already existing on the market with a basic knowledge of the vehicle models and a very small computational cost.